The nursery rhyme tells us that a star is “like a diamond in the sky.” In the past, we've always thought this was just a metaphor. But that phrase may be closer to the truth than we used to think: there may be actual diamonds in the sky!
We've known for decades that there are tiny diamonds elsewhere in our solar system. In some meteorites we see microscopic bits of diamond dust that are 25,000 times smaller than sand particles.
There are unusual clumps of millions of diamond crystals packed together into small porous rocks called carbonado, mostly in Brazil and central Africa. These are thought to have come from outer space about three billion years ago, back when South American and Africa were joined.
We've found tiny diamonds in half a dozen meteor craters around the globe. Pressures of more than a million atmospheres, generated by the impacts, suddenly compressed carbon atoms into crystalline patterns, creating minuscule diamonds.
More recently, we've found evidence for larger gems in more unusual places. The gas giant planets Uranus and Neptune have quite a bit of methane, a compound containing carbon, in their thick atmospheres. With sufficient heat and pressure, the carbon in the methane gas can be squeezed hard enough to form diamond raindrops that fall down toward the centers of the two planets.
These are all small bits of stuff, no larger than the stones in diamond rings. But recently we found a place where there may be a bigger hunk. There may be an entire star that's mostly diamond!
It's called BPM 37093, aka V886 Cen (variable star #886 in the constellation Centaurus). It's about 50 light years away (multiply by 26 trillion to get miles). You can't see it from South Jersey: you must travel farther south before it's above the horizon.
How do we get a star-sized diamond? It may a natural result of a star's life history. In stars, hydrogen gas fuses under pressure into helium gas, in the process releasing the vast energy that makes a star hot. Some stars, the heavier ones, can go further. If they get hot enough in their cores, the helium can start fusing into heavier elements, including carbon.
You can then have a huge mass of carbon, heavier than our Sun, under greater pressure than is found at the center of our star. Once fusion ceases, the star will shrink, and cool off. When it reaches a low enough temperature, around 19,000 F, the carbon may start crystallizing, forming a diamond.
This won't result in a brilliant faceted gemstone, of course. In such a huge lump of stuff, you can't maintain those well-organized crystal patterns that make a normal diamond sparkle. Moreover, there are still thin layers of helium and even some hydrogen overlaying the core, acting like oil smeared on a normal-sized gem. But it's still diamond!
How might we check what the inside of a star is like, to see if indeed the diamond did get formed? We can listen to the star chime!
As dying stars cool, eventually they reach a temperature when they naturally begin to pulsate, to “ring” like bells, in complex patterns. This happens to the Earth when an earthquake sends out waves of different kinds. By analyzing the results of siesmometers around the globe, we can determine what the interior of the Earth is like.
For stars, we look at those complicated pulsations (we don't actually listen to them), as shown by variations in the brightness of the star in different wavelengths. From those variations we can figure out quite a few details about what the interior of the star must be like. We've done a similar thing for the Sun for decades. The vibrations of BPM 37093 indicate the presence of a lot of crystallized material inside, and it has to be carbon.
Imagine the size of that piece of jewelry. In our everyday gem-measuring system, the star would weigh in at 10 billion trillion trillion carats! You'd need a jeweler's loupe the size of the Sun to grade it. If we ever find a way to travel to the stars, and can bring this gem back home, it will depress the diamond market like a falling stone. Be sure to sell all your diamond stock before then!